The present invention relates to the separation of gas from liquids in oil and gas wells, and particularly to methods of downhole separation of gas and liquid from a producing reservoir.
Production fluid, the fluid obtained from oil and gas wells, is generally a combination of substantially incompressible liquids and compressible gases. In particular, production fluid for methane production from coal formations includes such gases and water. Conventionally, pumping of such production fluid has presented difficulties due to the compressibility of the gases, which leads in the best of circumstances to reduction in pumping efficiency, and more detrimental, to pump lockage or cavitation.
Cavitation happens as cavities or bubbles form in pumped fluid, occurring at the low pressure or suction side of a pump. The bubbles collapse when passing to higher pressure regions, causing noise and vibration, leading to material erosion of the pump components. This can be expected to cause loss of pumping capacity and reduction in head pressure, reducing pump efficiency to the point of, over time, pump stoppage.
This has lead to the use of downhole gas and liquid separators to remove much of the compressible gasses from the production fluid prior to admission of the liquid component of the production fluid to the pump suction port. Gas separation conventionally is performed on production fluid at the bottom of the tubing string before pumping the liquid up the tubing, thereby improving efficiency and reliability of the pumping process. In some cases, waste components of the production fluid are re-injected above or below the production formation instead of bringing such waste components to the surface.
Examples of prior art downhole gas and liquid separators are taught by U.S. Pat. No. 5,673,752 to Scudder et al. (a separator that uses hydrophobic membrane for separation); U.S. Pat. No. 6,036,749 to Ribeiro et al. (a helical separator); U.S. Pat. No. 6,382,317 to Cobb (a powered rotary separator); U.S. Pat. No. 6,066,193 to Lee (inline separators with differently sized internal diameters); U.S. Pat. No. 6,155,345 to Lee et al. (a separator having flow-through bearings and multiple separation chambers); U.S. Pat. No. 6,761,215 to Morrison et al. (a rotary separator with a restrictor that creates a pressure drop as the fluid passes to the separation chamber); and U.S. Pat. No. 7,461,692 to Wang (multiple separation stages with each separation stage having a rotor with an inducer and impeller).
While many improvements have been taught by the prior art, there remains the need for efficient downhole gas separation that addresses the problems and shortcomings of such art, as the demands of the hostile environment of the downhole conditions of reservoir fluid at advanced pressures and elevated temperature conditions have continually been challenging. There is a need for downhole gas separation that can provide improved production rates while maintaining improved fluid lifting efficiencies over widely variable production conditions. It is to these improvements that the embodiments of the present invention are directed.